The Harmony folding boosters at Walmart are super narrow. DH has an 04 Accord and I have 2 radians and the folding booster in there. Radian and 2 of them would easily fit.

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Kristen
Middle school teacher by day, super mom by night
Mommy to The Boss~2007, The Energizer Bunny~2009, and The Princess~2011
My kids are no longer in diapers, but somehow, my computer keeps finding its way back here...

Harmony has boosters and even a combination seat that are all very narrow (in the 17" range). They have all gotten good reviews for safety and such. They were made to be a good seat that everyone can afford so they are only sold at Walmart right now. You would definitely be able to fit 3 across with those seats (well, seats with that width).

I have a ff britax roundabout 50 (was rf 3 across as well), harmony lite no back booster and a britax frontier 85 across a 2004 camry. It's tight and the girl (carpool) who rides in the carpool is only now figuring out how to buckle herself in (you have to sort of feel around for the buckle because it's hidden between the seats). I'm sure you could find narrower seats though because the britax are bulky.